Pagina-afbeeldingen
PDF
ePub

ASSAY AND ANALYSIS OF WHITE LEAD.* White lead may be assayed for colour and covering power by the usual methods (see Chapter X.).

Dry White Lead.—The purity of this pigment is ascertained by dissolving some of the lead in pure dilute nitric acid (1 acid, 2 water); strong nitric acid does not dissolve white lead owing to the insolubility of the lead nitrate which is formed in the acid; the ordinary commercial nitric acid contains sulphuric acid, which would lead to the formation of the insoluble sulphate of lead, the production of which might lead to the condemnation of a pure sample.

On adding dilute sulphuric acid to the solution, after diluting it with water and filtering off the precipitate of lead sulphate thus obtained, no further precipitate should be obtained on successively adding ammonia, ammonium sulphide, and ammonium oxalate to the filtrate.

A white precipitate with ammonium sulphide would indicate the presence of zinc white, which is a rare thing to find with white lead ; a white precipitate with ammonium oxalate would indicate the presence of whiting.

The insoluble residue, if any, will consist most probably of barytes, as other adulterants (for reasons already pointed out) are rarely used; still any lead sulphate, china clay, gypsum, or strontium sulphate which may be used would also be left as an insoluble residue on treating white lead with dilute nitric acid. To distinguish these bodies, boil the residue in hydrochloric acid and place the solution on one side to cool; if crystals of lead chloride separate out and the solution gives a white precipitate with barium chloride, then lead sulphate is present.

The hydrochloric acid solution should be diluted with water and sulphuretted hydrogen passed through it; the black precipitate of lead sulphide which may be obtained can be disregarded; this is filtered off and the filtrate boiled for some time to concentrate it and to drive off the sulphuretted hydrogen it contains. Then ammonia is added, when a white precipitate of alumina may be obtained indicating the presence of china clay ; this is filtered off, and to the filtrate is added ammonium carbonate, which will precipitate any calcium that may have been added, in the form of gypsum or whiting.

A little of the insoluble residue from the hydrochloric acid

By the term Assay as applied to pigments is understood their examination for all those properties of colour, covering power, &c., which give them value as pigments. These properties may be, and indeed often are, independent of the chemical composition of the pigments, as in the case of the ochres, umbers. Analysis is used in the ordinary sense of the term as applied in Chemistry.

*

a

should be held on a piece of platinum wire in the lower part of a Bunsen flame when, if it contains barytes, the flame will be coloured green; if strontium sulphate is present a crimson flame will be obtained. This test is not always easy to carry out, but with a little care the coloured flames can be obtained, and they are good proof of the presence of the pigments named. A quantitative analysis of white lead may be made as follows :

Weigh out 2 grammes and dissolve them in a beaker with the smallest possible quantity of pure dilute nitric acid, remove the insoluble matter by filtering, wash the residue well with warm water, adding the first wash waters to the filtrate, then dry the residue, place the filter paper and its contents in a weighed crucible and burn the paper; when completely burnt allow the crucible to cool in a desiccator and then weigh it. From the weight so obtained deduct 'the weight of the crucible and of the filter-paper ash, the difference is the weight of the insoluble residue.

To the filtrate add dilute sulphuric acid and a little alcohol, filter off the precipitate of lead sulphate which is obtained, wash it dry and burn it in a crucible as before. By multiplying the weight of the lead sulphate so obtained by 0·73554 the weight of lead oxide in the white lead can be found.

The carbonic acid can be ascertained by treating 2 grammes of the white lead with nitric acid in a Schrotter's or other form of apparatus for the estimation of carbonic acid.

The water may be determined by taking the difference between the amounts of lead oxide and carbonic acid thus found and 100.

Hygroscopic water can be ascertained by heating 2 grammes in an oven at 110° to 120° C. until no further loss of weight occurs.

If the white lead be adulterated with barytes, lead sulphate, china clay or some of the other insoluble white pigments, these will be left behind as an insoluble residue on treatment with nitric acid ; their amount is ascertained by filtering off, washing, drying, and burning the residue in a weighed crucible in the

Soluble adulterants like whiting, strontium carbonate, barium carbonate, and magnesite will be dissolved; if the presence of these is suspected, to the filtrate should be added more dilute sulphuric acid, which will precipitate the lead and barium ; this precipitate can be filtered off, the two can be separated by boiling with hydrochloric acid, which dissolves the lead sulphate but not the barium sulphate. To the filtrate ammonia and ammonium oxalate are added; this precipitates the calcium and the strontium, while the magnesite (if present) will remain in solution, and can be precipitated by sodium phosphate. It is not necessary to describe in detail the methods of separating these

usual way.

adulterants any further; some notes bearing on this point will be found in the descriptions of each individual pigment, while reference should be made to works on quantitative chemical analysis, such as that of Prof. Sexton, for fuller details.

By multiplying the weight of the carbonic acid by 5.05 the amount of lead oxide with which it is combined can be calculated; the two amounts added together give the quantity of lead carbonate in the white lead. Deducting the amount of lead oxide combined with the carbonic oxide from the total present, and multiplying this difference by 0·077, gives the amount of water combined with it to form lead hydroxide, and the two amounts added together gives the amount of the latter body present.

Paste White Lead can be quantitatively examined as follows :—Two grammes are treated with strong nitric acid at a gentle heat; this converts all the oil into an insoluble greasy matter, of which no account need be taken; the process then becomes identical with that for dry white lead. Should it be desired to ascertain the amount of the oil, then 10 grammes must be weighed into a filter paper and placed in a Soxhlet or other form of fat-extractor and the oil extracted by means of petroleum ether; the ethereal solution is then run into a weighed glass, the ether evaporated off and the oil weighed. If a fatextractor is not available it will suffice to agitate the white lead with some petroleum ether in a beaker, allowing the pigment to settle, pouring off the liquid into a weighed glass, again pouring on more ether, and again allowing the white lead to settle; the ether is poured off into the glass, or the mass may be filtered. Finally, the ether is evaporated off as before.

It is sometimes recommended to burn off the oil from the white lead in a crucible, but this course is not so satisfactory as treating it with petroleum ether, as not only is the oil burnt off, but the white lead is decomposed; whereas in the ether. method the lead is left in its original form for further examination if necessary

SULPHATE OF LEAD PIGMENTS.

Sulphate of lead, Pb S Oų, forms the basis of a number of white pigments which are made on a large scale and sold under a variety of names, such as Patent White Lead, Non-poisonous White Lead, Sublimed White Lead, &c. These do not consist entirely of lead sulphate but contain other bodies, such as zinc oxide, barytes, magnesia, &c., in varying quantities; they are

made by different methods, and most of those now sold are produced by patented processes.

Lead sulphate can be made by dissolving lead in strong sulphuric acid, the action is, however, but slight and does not form a commercial method of manufacture. It is mostly made by adding sulphuric acid to solutions of either lead acetate or lead nitrate; perhaps the best method is that where lead acetate is used.

Metallic lead is granulated by melting and pouring the molten lead into cold water; the object of granulating is to obtain the metal in such a form as to expose a large surface to the action of acids and air. The granulated lead is placed in a large tub fitted with a closed steam coil so that the action of the acid may be facilitated by heat, if necessary.

Acetic acid diluted with its own volume of water is poured on to the lead ; the action of the acid is at first rather sluggish, but by allowing the action to go on for about 12 hours, then running off the acid and leaving the lead in the tub without any liquor, a certain amount of oxidation goes on, resulting in the formation of a deposit of oxide on the lead, so that when the acid is again admitted to the lead, the acid acts more rapidly, and a strong solution of lead acetate is soon obtained ; this action is facilitated by a gentle heating of the contents of the tub. From the tub the solution is run into a large wooden vessel and to it is added strong sulphuric acid in small quantities at a time with constant stirring ; lead sulphate is thereby precipitated. Care is taken that the amount of sulphuric acid used is not sufficient to throw down all the lead, but that some of the latter is left in solution. The lead sulphate is allowed to settle, and then the clear supernatant liquor is pumped back again into the lead tub, for it contains all the acetic acid ; for, as will be seen from the equation

+

Pb 2 C2 H3 02
Lead acetate.

+ H, S 04

Sulphuric acid.

PbSO4
Lead sulphate.

2 H C2 H2 02
Acetic acid.

acetic acid is reproduced as the result of the reaction which goes on between the lead acetate and the sulphuric acid, and this acetic acid can be used over again for preparing fresh solution of lead for precipitation ; thus a comparatively small quantity of acetic acid may be used to prepare a large quantity of lead sulphate ; theoretically speaking, beyond the first charge no more acetic acid is required, but practically, there is a small loss which requires to be restored by new additions of acid from time to time.

:

The sulphate of lead which is precipitated is washed with water, drained on a filter, and dried; after which it is ready for use as a pigment, or can be combined with other white pigments if desired.

The acetic acid used should be as pure as possible; the usual commercial variety of a strength of about 1.050 = 10° to 12° Twaddell is sufficiently good for the purpose ; this contains about 35 per cent. of actual acetic acid, although some makes contain more. 100 lbs. of it will dissolve about 60 lbs. of lead supposing all the acetic acid exerts its solvent power, but in practice this never or but rarely happens, nor indeed is it necessary that it should hrana 2 to 28 lbs. of sulphuric acid will be required to po pocipitate the lead dissolved by this quantity of acetic acid. These figures are only approximate and are given simply as guides to actual practice. Lead sulphate has the formula Pb S 04, and contains Lead oxide, Pb ,

73.55 per cent. Sulphuric anhydride, sos, 26.45 and of metallic lead 68:31 per cent.

It is a white, somewhat crystalline, and very heavy powder, its specific gravity being about 6.3. It is only slightly soluble in water, insoluble in dilute acids and in alcohol, but soluble in solutions of ammoniacal salts, and in strong sulphuric acid; from the latter solution it is precipitateul on the addition of water. Boiling concentrated hydrochloric acid dissolves it, and crystals of lead chloride fall down as the solution cools.

As a pigment it is not satisfactory, its crystalline character reduces its body and covering power, causing it often to work streaky or livery under the brush ; this defect can be remedied to some extent by grinding it. It is not readily acted upon by sulphuretted hydrogen, and is, therefore, more permanent than white lead under exposure to air. Owing to its solubility being less it is free from the poisonous character of white lead, and, therefore, white pigments containing it are often sold as poisonous white leads.” Its colour or hue is a good white, but slightly yellower in tone than white lead and about equal to barytes.

It is used as a diluent in the manufacture of pale chromes. Many have been the attempts to make lead sulphate the base of commercial white leads, the records of these are to be found in the publications of the Patent Office, where they lie buried in an almost unknown condition, and it would really be most instructive for colour-makers and would-bie inventors if they

66

non

« VorigeDoorgaan »